In one conventional mode of "inching" a machine, the transmission is kept in gear and the brakes are manually modulated by use of a foot pedal. This is undesirable because it requires considerable operator effort and because the service brakes experience a relatively rapid rate of wear. In another well known mode, when the machine's service brakes are applied by the depression of a foot pedal, an associated disc and plate type transmission clutch is caused to slip so that the transmission is substantially neutralized. This is most often accomplished by the brake system fluid circuit acting upon an "inching" valve disposed between a pressure source and the clutch, as has been widely used in shuttle-type lift trucks. This is desirable since the machine's ground speed is reduced for more precise control, while the speed of the engine is maintained at a relatively high rate to enable a rapid response of the auxiliary equipment operated by the engine. However, controlled slip at a fixed setting of the manually operated control member in both of these modes of operation has heretofore been impractical.
In another conventional method of providing inching capabilities to a machine, an impeller clutch connected between a machine's engine and transmission is provided. Typically, the impeller clutch is actuated through an operator pedal. The operator pedal acts to engage and disengage the impeller clutch, thereby, varying the power transmitted by the drive train and slowing the machine. However, maintaining the machine at a speed proportional to the operator pedal is difficult due to changes in torque on the drivetrain.
The present invention is directed to overcoming one or more of the problems as set forth above.